1. Field of the Invention
This invention relates to an SIT (static induction transistor) gate driving circuit and more particularly to a switching circuit which can deliver the maximum switching characteristics of a depletion type SIT when it is used as a switching device.
2. Prior Art
The static induction transistor (hereinafter referred to as "SIT") is a kind of vertical field effect transistors (hereinafter referred to as "FET") and a multiple carrier device. It has superior characteristics as a switching device, since it does not cause secondary breakdown and does not have storage time which are defects inherent in bipolar transistors. However, this transistor has a high capacity between the drain and gate or the gate and source, so different from ordinary bipolar transistors, it is difficult to drive the SIT gate, and the switching performance of the SIT is greatly dependent on the gate driving method. As a result, the applications of SITs are extremely limited.
FIG. 1 is a cross-sectional view of a vertical junction FET. As shown in the figure, the source is located over the N-channel and the drain D is below the N-channel. In the middle, the gate G of a filament- or grid-like P-type semiconductor is formed. Since the SIT has the electrical characteristics of a depletion type, that is, "Normally ON" characteristics, when a voltage is applied between the drain D and the source S, a drain current flows unless a sufficient negative voltage is applied to the gate. Therefore, when the SIT is used as a switching device, it is necessary to apply a sufficient negative voltage to set the device in an non-operation state (OFF state).
As can be clearly shown in the sectional view of FIG. 1, the area between the drain D and the source S is equivalent to a capacitor with the channel area Ch as a dielectric in the OFF state. In the same way, the area between the gate G and the drain D and the area between the gate G and the source S form capacitors.
Accordingly, in the switching operation from the OFF to ON state, the charge stored in the capacitors must be discharged speedily to switch the device to the OFF state quickly. This discharging time is one of the greatest factors which prevents the switching-on time from being shortened. In the switching operation from the ON to OFF state, since current flows in the form of multiple carriers through the channel Ch during the ON state, the charge of these carriers must be taken out of the gate G to quickly switch the device from the ON to OFF state. This time required for taking out carriers is one of the greatest factors which prevents the switching-off time from being shortened in the same way as the discharging time required when the capacitor charge is discharged immediately after switching from the OFF to ON state.
FIG. 2 is a transformer coupling drive circuit embodying a known gate driving circuit. Since the circuit is driven by the transformer T1, this circuit has defects such as ringing or defective rising state during switching from the OFF to ON state of SIT1 and SIT2 due to effects of the coil of the driving transformer T1.
FIG. 3 shows an improved circuit embodying a known gate driving circuit. With this circuit, the pulse signal from the driving source 22 is amplified by the transistor Q21 and drives the driving transistor Q22 to further drive the SIT. To set the SIT to the OFF state, the transistor Q22 is turned ON and the negative voltage E23 is applied to the gate of the SIT. To set the SIT to the ON state, the transistor Q22 is turned OFF and the positive voltage E22 is applied to the gate through the resistor R25. With this driving circuit, during SIT switching from the ON to OFF state, the transistor Q22 turns ON to take out carriers from the gate of SIT, sufficiently shortening the OFF time. However, during SIT switching from the OFF to ON state, the charge stored in the area between the gate G and source S during the OFF period are discharged through the resistor R25. Accordingly, switching-on time is depended on the time constant determined by the resistor R25 and the electrostatic capacity C.sub.GS.
Therefore, switching-on time is prolonged unless a resistor R25 with a sufficiently low value is used. Furthermore, surge voltages, etc. may sometimes be generated by the load 21 depending on kinds of loads, and may appear at the gate through the capacity C.sub.GS between the drain and gate. This leaked surge voltage may damage the transistor Q22. Once the transistor Q22 is damaged, the negative voltage E23 is not applied. As a result, the SIT is set to the ON state all the time, and damaged in a short time due to excessive drain dissipation.